High Reliability in Wireless Networks Through Multi-Connectivity

While today's mobile networks are mainly used for connecting people, upcoming fifth generation (5G) technologies have the potential to expand wireless communication into a broad range of new applications and industries, with nearly everything being connected. Important parts of this vision are Ultra-Reliable Low-Latency Communication (URLLC) and the Tactile Internet, which enable controlling objects in real-time but also require unprecedented high reliability and low latency.
This thesis lays theoretical foundations for understanding and enhancing the reliability and availability of wireless networks. The primary approach followed is to model the statistical behavior of relevant components and to develop (semi-)-analytical tools for in-depth analysis of small outage probabilities. Initially, availability and survivability models are introduced for different combining schemes and small-scale fading channels. It is shown that, for achieving a particular availability, utilizing multiple low-power links can be considerably more power-efficient than using only a few powerful links. Moreover, novel closed-form expressions of the Minimum Duration Outage are derived for multiple selection-combined Rayleigh fading links.
The core contribution of this thesis is a signal-to-interference-plus-noise ratio (SINR) model that captures shadowing as a random component but considers a realistic user association based on total receive powers including shadowing. The SINR model enables a fast yet accurate analysis of the availability performance of wireless networks. Various system properties, e.g., multiple path loss models and antenna types, are considered for traditional sub-6 GHz carrier frequencies and higher frequencies in cmWave and mmWave bands. Results demonstrate that interference-limited sub-6 GHz carrier frequencies are, in principle, better suited to achieve the goals of URLLC, especially when interference mitigation techniques are applied in addition. Nonetheless, the availability of noise-limited higher carrier frequencies can be enhanced as well, for instance, by adapting the power spectral density. Moreover, intra- and inter-frequency multi-connectivity prove to be promising concepts for increasing the availability of wireless networks to a level suited for URLLC and Tactile Internet applications. Furthermore, the SINR model is combined with other causes of failure, namely mobility issues, non-radio failures, and small-scale fading, to a unifying framework for joint system analysis. The extension by mobility aspects provides valuable insights into how mobility deteriorates the availability performance and reveals that intra- and inter-frequency multi-connectivity have the potential to compensate for mobility effects.